* core: write test showing that TD is not stored properly at genesis

The ToBlock method applies a default value for an empty
difficulty value. This default is not carried over through the Commit
method because the TotalDifficulty database write writes the
original difficulty value (nil) instead of the defaulty value
present on the genesis Block.

Date: 2021-10-22 08:25:32-07:00
Signed-off-by: meows <b5c6@protonmail.com>

* core: write TD value from Block, not original genesis value

This an issue where a default TD value was not written to
the database, resulting in a 0 value TD at genesis.

A test for this issue was provided at 90e3ffd393

Date: 2021-10-22 08:28:00-07:00
Signed-off-by: meows <b5c6@protonmail.com>

* core: fix tests by adding GenesisDifficulty to expected result

See prior two commits.

Date: 2021-10-22 09:16:01-07:00
Signed-off-by: meows <b5c6@protonmail.com>

* les: fix test with genesis change
Co-authored-by: Martin Holst Swende <martin@swende.se>